Mixing apparatus and method



June 17, 1969 G. w. MCCURDY 3,450,389

' MIXING APPARATUS AND METHOD FiledAug. s, 1967 GORDON W. McCURDY INVENTOR.

BY ,m/M/

ATTORNEY United States Patent 3,450,389 MIXING APPARATUS AND METHOD Gordon W. McCurdy, Salt Lake City, Utah, assignor t0 Hercules Incorporated, Wilmington, Del., a corporation of Delaware Filed Aug. 8, 1967, Ser. No. 659,100 Int. Cl. Btllf 7/12, /06, 15/02 U.S. Cl. 25960 'Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process and apparatus for mixing flowable materials such as liquid and a particulate solid, together with a liquefying agent if desired, and particularly for mixing explosive materials such as the ingredients of a rocket propellant.

Mechanical mixers are very economical and eflicient in comparison with other types of mixers and have therefore been widely used. However, in mixing explosive materials with mechanical mixers, there is the danger of an accidental explosion resulting, for example, from friction or caviation in or throwing of the material being mixed, or impact of the mixing element with the side walls of the mixing chamber, or with a foreign object that finds its way into the mixture, or with broken parts such as an impeller blade. Another danger inherent in mechanical mixers is that explosive material in the form of dust or a slurry may find its way through packing and seals and may accumulate in the same or in other locations from which it is difficult to clean and wherein it may be accidentally ignited.

Accordingly, when used for mixing explosives, mechanical mixers must be extensively modified such as by increasing the clearance between the mixing element and the side walls of the mixing chamber, and by reducing the permissible variations or tolerances in the configuration of the mixing chamber as by grinding welds to provide a flush surface. Even when modified, mechanical mixers must usually be operated at reduced speeds when mixing explosive materials. All of these modifications in construction and operation reduce the economies and efiiciencies of mechanical mixers. However, despite the reduced economy and efiiciency, mechanical mixers are still employed in mixing explosive materials in preference to other mixing systems such as pumping, pouring or jet impingement which, although quite successful with low-vis cosity liquids, do not produce homogeneous mixtures within a reasonable time with the particular materials involved and without presenting their own explosion hazards.

The objects of this invention are therefore to provide a mixer especially adapted for mixing explosive materials, which mixer is simple, economical and efficient in that it can thoroughly mix a large quantity of material in a relative short period of time, and which at the same time has improved safety relative to the mixers presently being used. Further objects of this invention are to provide a mixer that is self-contained, thereby eliminating shafts and other moving parts projecting through the side wall of the mixing chamber and which require seals and packing,

all of which are subject to leakage and permit escape of the material and thereby present an explosion hazard or prevent maintaining a controllable atmosphere, for example, inert gas pressure or vacuum, within the mixing chamber.

The above objects have been attained by the provisions of a mixing chamber separated by divider means into two mixing compartments interconnected by a gap at the bottom thereof, each of said compartments having bottom walls sloping smoothly toward said gap. The material introduced into the mixing chamber is alternately forced through the gap from one mixing compartment to the other by a pressure differential between the two compartments.

Further objects of this invention will be apparent from an understanding of the preferred embodiment of the invention that is hereinafter described with reference to the accompanying drawing, which is a vertical sectional view through a mixer embodying the present invention.

With reference to the drawing, there is illustrated a mixing chamber 1 having a substantially cylindrical side wall 2 and a bottom wall 3 that consists of converging outer bottom wall section 4 and inner bottom wall section 5, the junction of which defines an annular bottom line 6 midway between the side wall 2 and the center 7 of the bottom wall 3 and at the lowest point in the bottom wall 3. The outer bottom wall section 4 extends smoothly downwardly and radially inwardly from the bottom 8 of the side wall 2 in the nature of an inverted frustrum of a cone having its base at the bottom 8 of the side wall 2. The inner bottom wall section 5 extends smoothly downwardly and outwardly from the center 7 of the bottom wall 3 in the nature of a cone having its apex at the center 7 and its base at the bottom line 6.

The top of the illustrated mixing chamber 1 is closed by a cover plate 9 that rests on a laterally extending flange 10 at the top of the side wall 2 and is releasably secured thereto as by a plurality of bolts 11 (only one of which is shown) pivotally mounted on brackets 12 at the underside of the flange 10. The bolts may extend upwardly through radial slots in the edges of the flange 10 and cover plate 9 and have nuts 13 threaded onto the free ends thereof.

Internally of the mixing chamber 1 there is a divider means in the form of a cylindrical sleeve 14 having a diameter substantially equal to the diameter of the annular bottom line 6. The sleeve 14 is secured to and depends from the cover 9 and has a length such that it terminates in a bottom edge 14a just short of the bottom line 6 to provide a gap 15. The sleeve 14 divides the mixing chamber 1 into two mixing compartments 16 and 17, the compartment 16 being the cylindrical space internally of the sleeve 14 and the compartment 17 being the annular space between the sleeve 14 and the side wall 2. The gap 15 interconnects the compartments 16 and 17 at the bottom line 6 and in eifect constitutes an annular orifice of substantially uniform width and of a length that is equal to the length of the compartments '16 and 17, which in the illustrated embodiment is the circumference of the sleeve 14.

Means are provided for establishing a pressure differential between the compartments 16 and 17, the illustrated means comprising a pressure conduit 18 for a pressurized fluid such as air and an exhaust conduit 19. The cover plate 9 is provided with a nipple 20 that communicates with the compartment 17 and a nipple 21 that communicates with the compartment 16. The nipple 20 is connected to a solenoid operated valve 22 that is adapted to be closed or to be connected selectively to the pressure conduit 18 or to the exhaust conduit 19. The nipple 21 is connected to a solenoid operated valve 23 that is also adapted to be closed or to be connected selectively to the exhaust conduit 19 or to the pressure conduit 18. The valves 22 and 23 are adapted to be operated by a time-controlled cyling device (not shown) or may be controlled manually or automatically for example by a device responsive to the level of the liquid in one of the individual compartments 16 and 17 of the mixing chamber.

In operation, the materials to be mixed are introduced into the mixing chamber 1 through the open top thereof. The top is then closed by the cover plate 9, and fluid under pressure is introduced into one of the mixing compartments 16 and 17 from the conduit 18 while the other compartment is connected to the exhaust conduit 19. The materials are thus forced from the pressurized compartment, through the gap 15, into the other compartment. At the desired point in the cycle, the pressure and exhaust connections are reversed and the materials are forced again through the gap 15 back into the original compartment. The connections are again reversed at the desired time, and this operation is repeated until a thorough and complete mixing is obtained. The compartments 16 and 17 are then disconnected from the pressure conduit 18, the cover plate 9 removed and the contents of the mixing chamber I removed as by dumping it from the open top or mouth of the mixing chamber.

The mixing action achieved by the above described apparatus has been found to be very effective and also very safe. The mixing occurs in what may be termed the mix ing spaces 24 and 25 which are the bottom portions of the mixing compartments 16 and 17, respectively. The stream of material from the gap 15 is discharged at a relatively high velocity into the mixing spaces 24 and 25 and, through a shearing action and through the turbulence that is generated, produces a thorough mixing of the material therein. The amount or rate of mixing is determined by the degree of agitation or turbulence that is generated which, in turn, depends upon the viscosity of the materials being mixed and the pressure differential, the latter of which can of course be controlled.

It has been found that a pressure differential of two pounds per square inch is sufficient to produce an effective mixing action with materials having a relatively low viscosity. With such a low pressure, power requirements are relatively low, massive equipment is not required and the materials used in making the mixing chamber can be selected for qualities other than strength, for example, safety in the presence of explosive materials, or low cost, or ease of fabrication.

One of the important features of this invention is in the configuration of the bottom wall 3 of the mixing chamber 1. The bottom wall of the mixing compartment 16 as well as the bottom wall 4 of the mixing compartment 17 slope smoothly downwardly toward the gap 15, or in other words, converge smoothly relatively to the sleeve 14. Thus, in passing from one of the mixing compartments 16 and 17 of the other, the material encounters a smoothly converging surface as it moves toward the gap 15 and then encounters a smoothly diverging surface as it moves into the other compartment. When the divider means 14 is disposed substantially at the transverse midpoint of the mixing chamber, that is, midway between the center and side walls with a mixing chamber that is cylindrical as illustrated, and the mixing compartments are thus of substantially equal width, the slope of the bottom walls 4 and 5 are substantially uniform. The advantages of such an arrangement are that there are no pockets or dead-spaces in which unmixed material can settle and there is a uniform mixing action. At the same time, by moving into the smoothly diverging mixing spaces 24 and 25, and until its energy is dissipated in the agitation or mixing of the material into which it passes, the impingement of the stream issuing from the gap 15 is uniformly distributed and uniformly effective upon all the material in the respective mixing space 24 or 25. In this manner, there is produced a thorough mixing of the material.

A further advantage of the present invention is that the pressure imposed on the material and thus the force impelling it from one compartment to the other is uniformly distributed along the gap 15, thereby minimizing the possibility of clogging or of a non-uniform mixing action.

While the gap 15 is herein illustrated as being annular or .angularly elongated, it will be obvious that a linearly elongated gap would also be effective. To avoid pockets, the gap 15 should be substantially coextensive with the length of the mixing compartments 16 and 17, that is, with the circumference of the sleeve 14 in a cylindrical mixing chamber or with the length from one end wall to the other in a linearly elongated mixing chamber. In order to provide a uniform mixing action, the gap 15 should also be substantially uniform in width, that is, the spacing of the edge 14a of the sleeve 14 relative to the bottom line 6 should be substantially uniform.

In contrast to other mixers, the herein disclosed mixer has numerous advantages. At no time is there any impacting of the mixture against itself as would occur for example in pumping a stream of the mixture into an accumulation of it, nor is it thrown against the wall of the mixing chamber. Inasmuch as there are no moving parts, there is no possibility of a moving element striking a stationary element, such as the wall of the mixing chamber or a foreign object or a broken-off part. Because of wear in moving parts, which could for example cause vibrations in apparatus for processing sensitive explosives, the initial cost as well as the maintenance of the mechanical mixer is greater. Without moving parts, there are no seals and other potential sources of leaking through which the material could pass to settle upon parts from which it would be diflicult to clean or on which it would constitute an explosion hazard. Being a closed system, dangers arising through the escape of volatile materials or explosive dust are also minimized. In some cases, the viscosity of the material may not be sufficiently low to permit it to be mixed by the above apparatus, in which case a liquefying agent may be added. The closed system minimizes the escape and loss of the liquefying agent and facilitates its eventual recovery. It will also be apparent that because of the ease of cleaning and low product loss, the present mixer has significant utility in mixing materials other than explosive materials.

While the above disclosure refers specifically to introducing pressure to cause the mixture to flow from one compartment to the other, it will be obvious that the required pressure differential could be established by drawing a vacuum on the one compartment. At the same time, the rate of flow can be controlled by bleeding atmospheric air into the opposite compartment. If desired, the vacuum system can be used to effect a de-aeration of the mixture as well as to move it from one compartment to the other. It is possible to impose a vacuum upon the material to de-aerate the same and thereafter switch to pressure to cause the material to flow between the two mixing compartments. Since the pressure is imposed only upon the surface of the material and does not include air bubbled through the material, there would be no significant re-absorption of air by the de-aterated material during the mixing operation. With the pressure conduit 18 and exhaust conduit 19 connected through the cover plate 9, the mixing chamber 1 can be readily opened or re-sealed through the fastening means for the cover plate 9.

The pressure differential for moving the mixture from one compartment to the other could also be obtained by sealing the one compartment after it has been pressurized and thereafter pressurizing and exhausting the other chamber. The trapped fluid is then further compressed as the mixture is forced into the closed compartment by pressurizing the other compartment arid expands to force the mixture out of the closed compartment when the other compartment is exhausted. An advantage of this arrange ment is that the problem of dust is virtually eliminated by adding the dry ingredient to the closed compartment.

It is also within the contemplation of this invention that the pressure differential may he graduated as the material moves from one compartment to the other, or, because of the variation in the head as the level of material decreases in the one compartment and increases in the other, that the imposed pressure may be graduated to maintain a substantially constant pressure at the gap 15.

What I claim and desire to protect by Letters Patent 1s:

1. Mixing apparatus comprising a closed mixing chamber having a bottom wall consisting of bottom wall sections converging to a bottom line extending lengthwise of and being substantially co-extensive longitudinally with said mixing chamber and constituting the transverse lowpoints along said mixing chamber, divider means for dividing said mixing chamber into two individual comd partments, said divider means having a bottom edge disposed in spaced substantially parallel relation to said bottom line to provide a narrow gap interconnecting said compartments and being disposed between said bottom wall sections whereby each of said bottom wall sections constitutes a bottom wall for one of said compartments that slopes smoothly downwardly toward said gap, means for introducing material to be mixed into said mixing chamber, means for establishing a pressure differential between said compartments for forcing the material from the compartment of higher pressure through said gap into the compartment of lower pressure, and means for cyclically reversing said pressure differential and thus the flow of the material after a pre-determined amount of the material has passed through said gap from the one compartment to the other.

2. Mixing apparatus in accordance with claim 1 in which said mixing chamber has a substantially cylindrical side wall and said divider means comprises a sleeve disposed concentrically within said mixing chamber between said side wall and the axis of said side wall whereby said mixing chamber comprises an annular outer compartment and a cylindrical inner compartment.

3. Mixing apparatus in accordance with claim 2 in which said sleeve is disposed substantially midwaybetween said side wall and the axis thereof whereby the bottom walls of each of said compartments slope unitormly toward said gap.

4. Mixing apparatus in accordance with claim 1 in which said means for cyclically reversing said pressure differential is time controlled.

5. Mixing apparatus in acordance with claim 1 in which said mixing chamber is closed at the top by a removable cover plate.

6. Mixing apparatus in accordance with claim 5 in which said pressure differential is established by air under pressure introduced into one of said compartments through said cover plate while the other of said compartments is exhausted through connections made through said cover plate.

7. Mixing apparatus in accordance with claim 1 in which said pressure diflerential is established by introducing air under pressure into one of said compartments while exhausting the other of said compartments.

8. Mixing apparatus in accordance with claim 1 in which said pressure differential is established by drawing a vacuum on one of said compartments while the other of said compartments is exposed to fluid at least at atmospheric pressure.

9. A process for mixing a plurality of materials in liquid state comprising placing said materials in a mixing chamber having a pair of mixing compartments interconnected by an elongated gap extending the length of said compartments and disposed at the bottom line thereof, each of said compartments having a bottom wall sloping downwardly smoothly toward said gap, sealing said mixing chamber, establishing a pressure difierential between said mixing compartments whereby said material will flow through said gap from the compartment of higher pressure to the compartment of lower pressure, reversing said pressure diiferential at a pre-selected point in the cycle to cause said material to flow back through said gap, and repeating reversal of said pressure differential until the desired mixing level is achieved.

10. A process for mixing in accordance with claim 9 in which the pressure differential is established by applying fluid pressure to one of said compartments while the other of said compartments is exhausted.

References Cited FOREIGN PATENTS 1,136,984 9/1962 Germany.

WALTER A. SCHEEL, Primary Examiner.

US. Cl. X.R. 

